1. Field of the Invention
The present invention relates to a digital communications method that enables one-way multi-mode data transmission with low error characteristics. It particularly relates to a multi-mode block-coded modulation/demodulation method that utilizes unequal and equal error protection in order to send transmissions in which most-significant bits are mixed with other bits.
2. Description of the Prior Art
A first example of the prior art is shown in FIG. 1, which is a block diagram of a conventional one-way adaptive transmission method. In FIG. 1, the preamble carries information such as the number of transmission bits and the data transmission rate. On the receiving side the preamble is read and the necessary number of bits decoded. In the one-way adaptive transmission method of FIG. 1, the preamble is used to adaptively change how the data portion is modulated and decoded. Thus, the method requires the addition to the preamble of information such as the data partition system and the like. If 1) there are errors in the coded information, the data part cannot be correctly decoded even if it is received normally; 2) if preamble coding is used to avoid this, it increases the length of the preamble, decreasing the transmission efficiency; and 3) on the receiving side, in order to decode the received data, it is necessary to provide a plurality of modulators to decode the preamble and switch decoders.
FIG. 2 shows a second example of the prior art. This is a one-way adaptive method in which, depending on the location of the signal points, the modulation system is adaptively changed. In this case, quadrature amplitude modulation (16QAM) and quadrature phase shift keying (QPSK) signal points are used simultaneously to enable series of different bit numbers to be transmitted depending on whether the transmission symbols are sent as 16QAM or as QPSK points, and does not require preamble insertion. In this system, however, the Euclidean distance (d1, d2) between pluralities of adjacent signal points becomes d1<d2, so that in the case of two-bit transmissions, there is a marked degradation in the bit error rate compared to QPSK, so the system has the drawback of lacking practical utility.
Moreover, while not illustrated, as a third example of the prior art, there is the conventional multi-level coded modulation system in which a fixed transmission system is used in designated communications. There are also mobile station communication systems used in mobile unit communications that use a type of phase modulation such as QPSK and fixed data transmission rates.
As described above, the prior art includes problems such as: 1) if there are errors in the coded information the data portion cannot be correctly decoded even if it is received normally; 2) using preamble coding to prevent this happening increases the length of the preamble, thereby decreasing the transmission efficiency; and 3) on the receiving side, in order to decode the received data, it is necessary to provide a plurality of modulators to decode the preamble and switch decoders, resulting in a complex configuration. Also, in the case of a one-way adaptive system in which the modulation method is adaptively changed according to the location of the signal points, a problem is that with two-bit transmissions there is a marked degradation in the bit error rate compared to QPSK, making the system impractical. Moreover, in the case of a multi-level encoding system, a fixed transmission system is used even for mobile station transmission systems, and so a problem is that, in order to maintain the bit error rate, it is not possible to switch communication systems according to the transmission route.
To resolve the above problems, an object of the present invention is to provide a multi-mode block-coded modulation/demodulation method that can be used in the fields of coding systems, modulation systems and coded modulation systems to adaptively change transmission characteristics.
Here, multi-mode transmission refers to transmission effected by changing the component elements of the multi-level coded modulation system according to transmission mode; such elements include the number of code levels, the symbols at each level, the set-partitioning method, modulation method, and so forth. In addition to multi-mode transmission, by combining the block-coded modulation system with Viterbi decoding, the present invention enables the achievement of a high coding gain and good transmission quality.